Respiratory complications are the most common cause of morbidity and mortality following spinal cord injury (SCI). These complications arise partly due to the loss of supraspinal control over the expiratory muscles, such as abdominal and lower intercostal muscles. Disrupted neuromuscular central nervous system control of the expiratory muscles interferes with producing an effective cough to clear airway secretions, resulting in respiratory tract infections.
Current management of expiratory dysfunction in subjects with SCI includes passive postural drainage, tracheal suctioning, and assisted or “quad” cough. Each method has a varying degree of effectiveness and all require active assistance. Further, functional electrical stimulation (FES) has also been utilized to produce an expiratory function by direct stimulation of the spinal cord. FES is a technique that uses electrical currents to activate nerves innervating extremities affected by paralysis resulting from spinal cord injury (SCI), head injury, stroke or other neurological disorders. For example, FES was shown to produce high positive airway pressures (Paw) in animal studies when a plate electrode was applied to the epidural surface of the lower thoracic spinal cord in the animal (DiMarco et al., Am. J. Respir. Crit. Care Med. 151: 1466-1471, 1995). Maximal expiratory pressure generation occurred when stimulation was applied in the area of T9-T10 thoracic vertebrae of the animal. In a similar human study, simultaneous stimulation at the T9 and L1 levels of a tetraplegic subject produced a maximal Paw of 132H2O and peak expiratory flow of 7.4 L/s (DiMarco et al., Am. J. Respir. Crit. Care Med. 173(12): 1386-1389, 2006).
Functional magnetic stimulation (FMS) has also been demonstrated to produce an expiratory function by stimulating the lower thoracic spinal nerve root in a human subject (Lin et al., J. Appl. Physiol. 84: 1144-1150, 1998; Lin et al., Arch. Physic. Med. Rehabil. 79: 517-22, 1998). FMS is achieved by an external application of electromagnetic energy to the spinal cord. For example, maximal expiratory pressure generated by FMS was 83.6±16.4 cm H2O when a magnetic coil was placed at T9 spinous process in able-bodied subjects (Lin et al., J. Appl. Physiol. 84: 1144-1150, 1998). Further, when a similar stimulation protocol was applied to subjects with SCI, FMS of the expiratory muscles also produced a substantial expiratory pressure of 68.2±24.1 cm H2O in which placement of the magnetic coil at the T10-T11 spinous process produced the highest expiratory pressure and flow (Id.).
It would be advantageous to provide improved methods of minimally-invasive neuromuscular stimulation to produce expiratory function without requiring major surgery in order to minimize infection and facilitate maintenance of the stimulation device. The prior direct implantation of epidural FES electrodes require surgical exposure of the central nervous system, with attendant risks of infection and additional trauma. Although FMS avoids those drawbacks, it requires the use of inconvenient external magnets and associated equipment.